U.S. patent number 9,178,137 [Application Number 13/963,682] was granted by the patent office on 2015-11-03 for magnetoresistive element and magnetic memory.
The grantee listed for this patent is Youngmin Eeh, Daisuke Ikeno, Tadashi Kai, Toshihiko Nagase, Katsuya Nishiyama, Daisuke Watanabe. Invention is credited to Youngmin Eeh, Daisuke Ikeno, Tadashi Kai, Toshihiko Nagase, Katsuya Nishiyama, Daisuke Watanabe.
United States Patent |
9,178,137 |
Eeh , et al. |
November 3, 2015 |
Magnetoresistive element and magnetic memory
Abstract
A magnetoresistive element includes first and magnetic layers,
first and second non-magnetic layers and a W layer. Each of the
first and second magnetic layers includes an axis of easy
magnetization in a direction perpendicular to a film plane. The
first magnetic layer has a variable magnetization direction. The
second magnetic layer has an invariable magnetization direction.
The first non-magnetic layer is provided between the first and
second magnetic layers. The second non-magnetic layer is arranged
on a surface of the first magnetic layer opposite to a surface on
which the first non-magnetic layer is arranged and contains MgO.
The W layer is arranged on a surface of the second non-magnetic
layer opposite to a surface on which the first magnetic layer is
arranged, and is in contact with the surface of the second
non-magnetic layer.
Inventors: |
Eeh; Youngmin (Kawagoe,
JP), Nishiyama; Katsuya (Yokohama, JP),
Ikeno; Daisuke (Yokkaichi, JP), Nagase; Toshihiko
(Tokyo, JP), Kai; Tadashi (Tokyo, JP),
Watanabe; Daisuke (Kai, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Eeh; Youngmin
Nishiyama; Katsuya
Ikeno; Daisuke
Nagase; Toshihiko
Kai; Tadashi
Watanabe; Daisuke |
Kawagoe
Yokohama
Yokkaichi
Tokyo
Tokyo
Kai |
N/A
N/A
N/A
N/A
N/A
N/A |
JP
JP
JP
JP
JP
JP |
|
|
Family
ID: |
51568443 |
Appl.
No.: |
13/963,682 |
Filed: |
August 9, 2013 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20140284539 A1 |
Sep 25, 2014 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61804478 |
Mar 22, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
43/12 (20130101); H01L 43/08 (20130101); H01L
27/226 (20130101) |
Current International
Class: |
H01L
43/08 (20060101); H01L 43/12 (20060101); H01L
27/22 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
H Sato, et al., "Perpendicular-anisotropy CoFeB--MgO magnetic
tunnel junctions with a MgO/CoFeB/Ta/CoFeB/MgO recording
structure", Applied Physics Letters 101, 022414 (2012), four pages
(in English). cited by applicant.
|
Primary Examiner: Bernstein; Allison P
Attorney, Agent or Firm: Holtz, Holtz, Goodman & Chick
PC
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application
No. 61/804,478, filed Mar. 22, 2013, the entire contents of which
are incorporated herein by reference.
Claims
What is claimed is:
1. A magnetoresistive element comprising: a first magnetic layer
which includes an axis of easy magnetization in a direction
perpendicular to a film plane and a variable magnetization
direction; a second magnetic layer which includes an axis of easy
magnetization in the direction perpendicular to the film plane and
an invariable magnetization direction; a first non-magnetic layer
provided between the first magnetic layer and the second magnetic
layer; a second non-magnetic layer arranged on a surface of the
first magnetic layer opposite to a surface on which the first
non-magnetic layer is arranged, the second non-magnetic layer
containing MgO; and a W layer arranged on a surface of the second
non-magnetic layer opposite to a surface on which the first
magnetic layer is arranged, the W layer being in contact with the
surface of the second non-magnetic layer, and the W layer including
an amorphous state.
2. The magnetoresistive element according to claim 1, further
comprising: a CoFeB layer arranged on a surface of the W layer
opposite to a surface on which the second non-magnetic layer is
arranged, the CoFeB layer being in contact with the surface of the
W layer.
3. The magnetoresistive element according to claim 1, wherein the
first non-magnetic layer contains MgO.
4. The magnetoresistive element according to claim 3, wherein the
second non-magnetic layer contains oxygen per unit volume in an
amount less than that of the first non-magnetic layer.
5. The magnetoresistive element according to claim 1, wherein the
first magnetic layer includes a first CoFe-containing layer, and a
second CoFe-containing layer, the first CoFe-containing layer
contacts the second non-magnetic layer, and the second
CoFe-containing layer contacts the first non-magnetic layer.
6. The magnetoresistive element according to claim 1, wherein the
MgO contained in the second non-magnetic layer contains oxygen in
an amount less than that of MgO with a stoichiometric
composition.
7. A magnetoresistive element comprising: a first magnetic layer
which includes an axis of easy magnetization in a direction
perpendicular to a film plane and a variable magnetization
direction; a second magnetic layer which includes an axis of easy
magnetization in the direction perpendicular to the film plane and
an invariable magnetization direction; a first non-magnetic layer
provided between the first magnetic layer and the second magnetic
layer; a second non-magnetic layer arranged on a surface of the
first magnetic layer opposite to a surface on which the first
non-magnetic layer is arranged, the second non-magnetic layer
containing MgO; and a Ti layer arranged on a surface of the second
non-magnetic layer opposite to a surface on which the first
magnetic layer is arranged, the Ti layer being in contact with the
surface of the second non-magnetic layer, and the Ti layer
including a polycrystalline state.
8. The magnetoresistive element according to claim 7, wherein the
first non-magnetic layer contains MgO.
9. The magnetoresistive element according to claim 8, wherein the
second non-magnetic layer contains oxygen per unit volume in an
amount less than that of the first non-magnetic layer.
10. The magnetoresistive element according to claim 8, wherein the
first magnetic layer includes a first CoFe-containing layer and a
second CoFe-containing layer, the first CoFe-containing layer
contacts the second non-magnetic layer, and the second
CoFe-containing layer contacts the first non-magnetic layer.
11. The magnetoresistive element according to claim 8, wherein the
first magnetic layer contains a CoFe-containing layer, a first
surface of the CoFe-containing layer contacts the second
non-magnetic layer, and a second surface opposite to the first
surface of the CoFe-containing layer contacts the first
non-magnetic layer.
12. The magnetoresistive element according to claim 7, wherein the
MgO contained in the second non-magnetic layer contains oxygen in
an amount less than that of MgO with a stoichiometric composition.
Description
FIELD
Embodiments described herein relate generally to a magnetoresistive
element and a magnetic memory.
BACKGROUND
In recent years, magnetic random access memories (hereinafter
abbreviated MRAM), which use a magnetoresistive effect of a
ferromagnetic substance, have been attracting attention as
next-generation solid-state nonvolatile memories capable of reading
and writing data at high speed, offering large-capacity storage,
and operating on low power consumption. In particular,
magnetoresistive elements including a ferromagnetic tunneling
junction have been gaining attention since such magnetoresistive
elements were discovered to exhibit a high magnetoresistance rate.
A ferromagnetic tunneling junction has a three-layer stacked
structure including a storage layer whose magnetization direction
is variable, an insulating material layer, and a fixed layer facing
the storage layer and maintaining a predetermined magnetization
direction.
The magnetoresistive element including a ferromagnetic tunneling
junction is also called a magnetic tunnel junction (MTJ) element,
and a writing (spin-transfer torque writing) method which uses a
spin-momentum transfer (SMT) has been proposed as a writing method
thereof. A perpendicular magnetization film, which includes an axis
of easy magnetization in a direction perpendicular to a film plane,
has been considered to be used as a ferromagnetic material forming
the magnetoresistive element.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view illustrating a structure of a
magnetoresistive element according to a first embodiment.
FIG. 2 is a cross-sectional view illustrating a structure from a
buffer layer to a tunnel barrier layer on a semiconductor substrate
in the magnetoresistive element.
FIGS. 3, 4 and 5 are cross-sectional views illustrating a method of
manufacturing an underlying layer (MgO) in the magnetoresistive
element.
FIG. 6 is a cross-sectional view illustrating a structure from a
buffer layer to a tunnel barrier layer on a semiconductor substrate
according to a second embodiment.
FIG. 7 is a graph comparing perpendicular magnetic anisotropy
energy between the storage layer of the embodiments and a
conventional storage layer.
FIG. 8 is a circuit diagram illustrating a configuration of an MRAM
according to a third embodiment.
DETAILED DESCRIPTION
Hereinafter, a magnetoresistive element and a magnetic memory
according to embodiments will be described with reference to the
accompanying drawings. In the description that follows, the
structural elements having the same function and configuration will
be denoted by the same reference numbers and repeated description
will be given only when necessary.
In general, according to one embodiment, a magnetoresistive element
includes a first magnetic layer, a second magnetic layer, a first
non-magnetic layer, a second non-magnetic layer, and a W layer. The
first magnetic layer includes an axis of easy magnetization in a
direction perpendicular to a film plane and a variable
magnetization direction. The second magnetic layer includes an axis
of easy magnetization in the direction perpendicular to the film
plane and an invariable magnetization direction. The first
non-magnetic layer is provided between the first magnetic layer and
the second magnetic layer. The second non-magnetic layer is
arranged on a surface of the first magnetic layer opposite to a
surface on which the first non-magnetic layer is arranged. The
second non-magnetic layer contains MgO. The W layer is arranged on
a surface of the second non-magnetic layer opposite to a surface on
which the first magnetic layer is arranged. The W layer is in
contact with the surface of the second non-magnetic layer.
First Embodiment
A description will be given on a magnetoresistive element according
to a first embodiment.
[1] Structure of Magnetoresistive Element
In the present specification and the claims, the term
magnetoresistive element refers to a magnetic tunnel junction (MTJ)
element in which a semiconductor or an insulating material is used
as a tunnel barrier layer. In the cross-sectional views of FIG. 1
and subsequent drawings, only the main parts of the
magnetoresistive element are shown; however, other layers may also
be included as long as the illustrated structures are included.
FIG. 1 is a cross-sectional view illustrating a structure of the
magnetoresistive element according to the first embodiment.
A magnetoresistive element 100 performs writing by a spin-transfer
torque magnetization reversal method. That is, information is
stored by changing relative angles of magnetization between a
storage layer and a reference layer to a parallel state and an
antiparallel state (i.e., minimum resistance and maximum
resistance) and associating the states with binary 0 or 1,
according to a direction of a spin-polarized current flowing in a
direction perpendicular to a film plane relative to each layer.
As shown in FIG. 1, the magnetoresistive element 100 at least
comprises two magnetic layers: a magnetic layer (storage layer) 1;
and a magnetic layer (reference layer) 2, and a non-magnetic layer
3 provided between the magnetic layer 1 and the magnetic layer
2.
The magnetic layer 1 is provided on an underlying layer 4, and
includes an axis of easy magnetization in a direction perpendicular
to a film plane. A magnetization direction of the magnetic layer 1
is variable. The magnetization direction being variable means that
the magnetization direction changes before and after writing. In
the present specification, the term film plane refers to an upper
surface of a target layer. Hereinafter, the magnetic layer 1 will
be referred to as a storage layer (also a free layer, a
magnetization free layer, a magnetization variable layer, or a
recording layer). Magnetization in a direction perpendicular to the
film plane will be referred to as perpendicular magnetization.
The storage layer 1 has a structure in which a first storage layer
(CoFeB layer) 11, an insertion layer (W or Ta layer) 12, and a
second storage layer (FeCoB layer) 13 are stacked in this order on
the underlying layer 4.
The magnetic layer 2 includes an axis of easy magnetization in a
direction perpendicular to the film plane, and a magnetization
direction of the magnetic layer 2 is invariable or fixed with
respect to the storage layer 1. The magnetization direction being
invariable means that the magnetization direction does not change
before and after writing. Hereinafter, the magnetic layer 2 will be
referred to as a reference layer (also a fixed layer, a
magnetization fixed layer, a pinned layer, a standard layer, and a
magnetization standard layer).
The reference layer 2 has a structure in which an interface
reference layer (CoFeB layer) 21, a function layer (Ta layer) 22, a
Co layer 23, and an artificial lattice layer ([Co/Pt]n) 24 are
stacked in this order on the non-magnetic layer 3.
Designations of structures that can be regarded essentially equal
to those of the present embodiment are not limited to those
mentioned above.
The non-magnetic layer 3 is also called a tunnel barrier layer, and
is formed of an insulating film of an oxide, such as MgO. In the
description that follows, the non-magnetic layer 3 will be referred
to as a tunnel barrier layer.
In the magnetoresistance element 100 of the present embodiment, a
buffer layer 6 is formed on a semiconductor substrate 5, and the
underlying layer 4 is formed on the buffer layer 6. Further, the
storage layer 1, the tunnel barrier layer 3, and the reference
layer 2 are sequentially formed on the underlying layer 4. A
detailed structure and characteristics of the buffer layer 6 and
the underlying layer 4 will be described later.
A spacer layer (Ru layer, for example) 7 is formed on the reference
layer 2. A shift-canceling layer 8 is formed on the spacer layer 7.
The shift-canceling layer 8 relaxes and adjusts a shift of a
switching current of the storage layer 1 caused by a leakage
magnetic field from the reference layer 2.
A cap layer 9 is formed on the shift-canceling layer 8. A hard mask
10 is formed on the cap layer 9. The cap layer 9 and the hard mask
10 function mainly as protective layers which prevent oxidation of
the magnetic layers, for example. The cap layer 9 has a structure
in which a Pt layer, a W layer, and a Ru layer are stacked in this
order on the shift-canceling layer 8.
The magnetoresistive element 100 of the first embodiment shown in
FIG. 1 has a top-pin structure, in which the storage layer 1 is
formed on the underlying layer 4, and the tunnel barrier layer 3
and the reference layer 2 are formed on the storage layer 1. The
present embodiment may also take a bottom-pin structure, in which
the reference layer 2 is formed on the underlying layer 4, and the
tunnel barrier layer 3 and the storage layer 1 are formed on the
reference layer 2.
Next, writing and reading operations in the magnetoresistive
element 100 of the present embodiment will be briefly
discussed.
The magnetoresistive element 100 is a magnetoresistive element used
in a spin-transfer torque writing method. That is, in a writing
operation, by letting a current flow in a direction perpendicular
to the film plane from the reference layer 2 to the storage layer 1
or from the storage layer 1 to the reference layer 2, electrons
containing spin information are injected into the storage layer 1
from the reference layer 2. Since a spin angular momentum of the
injected electrons is transferred to electrons of the storage layer
1 according to the law of conservation of spin angular momentum,
magnetization of the storage layer 1 is reversed.
For example, when a magnetization direction of the storage layer 1
and a magnetization direction of the reference layer 2 are
antiparallel, a current flows from the storage layer 1 to the
reference layer 2. In this case, electrons flow from the reference
layer 2 to the storage layer 1. The electrons spin-polarized by the
reference layer 2 flow to the storage layer 1 through the tunnel
barrier layer 3, the spin angular momentum is transferred to the
storage layer 1, and the magnetization direction of the storage
layer 1 is reversed so as to be parallel to the magnetization
direction of the reference layer 2.
When the magnetization direction of the storage layer 1 and the
magnetization direction of the reference layer 2 are parallel, on
the other hand, a current flows from the reference layer 2 to the
storage layer 1. In this case, electrons flow from the storage
layer 1 to the reference layer 2. The electrons spin-polarized by
the storage layer 1 flow to the reference layer 2 through the
tunnel barrier layer 3, and electrons having the same spin as the
magnetization direction of the reference layer 2 pass through the
reference layer 2, while electrons having a spin opposite to the
magnetization direction of the reference layer 2 are reflected off
an interface between the tunnel barrier layer 3 and the reference
layer 2 and flow to the storage layer 1 through the tunnel barrier
layer 3. As a result, the spin angular momentum is transferred to
the storage layer 1 and the magnetization direction of the storage
layer 1 is reversed so as to be antiparallel to the magnetization
direction of the reference layer 2.
When information is read from the magnetoresistive element 1, a
read current which does not reverse magnetization of the storage
layer 1 flows between the storage layer 1 and the reference layer 2
through the tunnel barrier layer 3. Thereby, information can be
read from the magnetoresistive element 100.
[2] Structures of Buffer Layer, Underlying layer, and Storage
Layer
Next, structures of the buffer layer 6, the underlying layer 4, and
the storage layer 1 according to the present embodiment will be
described in detail.
FIG. 2 is a cross-sectional view illustrating a structure from the
buffer layer 6 to the tunnel barrier layer 3 on the semiconductor
substrate 5.
The buffer layer 6, the underlying layer 4, the storage layer 1,
and the tunnel barrier layer 3 are sequentially formed on the
semiconductor substrate 5.
The buffer layer 6 has a structure in which a first buffer layer 61
and a second buffer layer (a CoFeB layer 62 and a W layer 63) are
stacked in this order on the semiconductor substrate 5. The first
buffer layer 61 is formed of W, Ta, Hf, or the like. The thickness
of the first buffer layer 61 is approximately 10-100 .ANG..
The CoFeB layer 62 is formed of (CoFe.sub.0-100)B.sub.20-50. That
is, the CoFeB layer 62 has a composition in which B is contained at
a ratio of 20-50 at %, and a ratio of Co to Fe falls within the
range of 1:1 to 0:1. The thickness of the CoFeB layer 62 is
approximately 5-10 .ANG..
The W layer 63 should preferably be amorphous, and has a thickness
of approximately 5-20 .ANG.. The W layer 63 maintains an amorphous
state until the thickness reaches approximately 20 .ANG.. The W
layer 63 does not necessarily need to be amorphous, and may also be
monocrystalline or polycrystalline.
The underlying layer 4 includes an MgO layer formed on the W layer
63 of the buffer layer 6. The MgO layer is formed of MgO in an
oxygen-deficient state, which contains oxygen in an amount less
than that of MgO with a stoichiometric composition. That is, an
amount of oxygen per unit volume contained in the MgO layer of the
underlying layer 4 is less than that of the MgO forming the tunnel
barrier layer 3.
The storage layer 1 has a structure in which the first storage
layer (CoFeB layer) 11, the insertion layer (W or Ta layer) 12, and
the second storage layer (CoFeB layer) 13 are stacked in this order
on the underlying layer 4. The CoFeB layers 11, 13 are formed of
(CoFe.sub.50-100) B.sub.10-20. That is, the CoFeB layer 11 has a
composition in which B is contained at a ratio of 10-20 at %, and a
ratio of Co to Fe falls within the range of 0.5:0.5 to 0:1. The
thickness of the CoFeB layer 11 is approximately 5-20 .ANG.. The
thickness of the CoFeB layer 13 is approximately 5-15 .ANG.. The
thickness of the insertion layer 12 is approximately 1-5 .ANG.. In
the above-described example, the storage layer 1 has a stacked
structure, but may also be a single CoFeB layer.
A tunnel barrier layer 3 formed of an MgO layer is arranged on the
CoFeB layer 13 of the storage layer 1.
By using the structure shown in FIG. 2, perpendicular magnetic
anisotropy can be generated in the CoFeB layers 13, 11 in the
vicinity of an interface between the MgO layers 3, 4 provided on
both surface sides of the storage layer 1 and the storage layer 1.
Further, the resistance of the underlying layer (MgO layer) 4 can
be reduced to 0.5 .mu..OMEGA./cm.sup.2 or less.
In the present embodiment, an MgO layer in an oxygen-deficient
state is formed as the underlying layer 4 on the W layer 63 of the
buffer layer 6. Thereby, the series resistance of the underlying
layer (MgO layer) 4 arranged between the buffer layer 6 and the
storage layer 1 can be greatly reduced.
A method of forming MgO in an oxygen-deficient state as the
underlying layer 4 will be described below.
FIGS. 3, 4 and 5 are cross-sectional views illustrating a method of
manufacturing MgO as the underlying layer 4.
As shown in FIG. 3, an Mg layer 41 is formed on the buffer layer 6
by sputtering, for example, so as to have a thickness of
approximately 2-10 .ANG..
After that, as shown in FIG. 4, an MgO layer 41A is formed by
oxidizing the Mg layer 41. After that, an Mg layer 42 is formed on
the MgO layer 41A by sputtering, so as to have a thickness of 3-15
.ANG.. The thickness of the Mg layer 42 should preferably be equal
to or greater than the thickness of the Mg layer 41.
Thereby, oxygen in the MgO layer 41A is diffused into the Mg layer
42, and an MgO layer 4 in an oxygen-deficient state is formed as
shown in FIG. 5.
In the above-described example, an MgO layer in an oxygen-deficient
state is formed by oxidizing an Mg layer and stacking an Mg layer
thereon; however, MgO in an oxygen-deficient state may be directly
deposited on the buffer layer 6 by sputtering using MgO in an
oxygen-deficient state as a target.
Second Embodiment
In the first embodiment, an example of forming a W layer on a
surface of a buffer layer has been described. In a second
embodiment, on the other hand, an example of forming a Ti layer on
a surface of the buffer layer will be described. Since an overall
structure of the magnetoresistive element is the same as that of
the first embodiment, a description thereof will be omitted.
[1] Structures of Buffer Layer, Underlying layer, and Storage
Layer
A detailed description will be given on structures of a buffer
layer 6, a underlying layer 4, and a storage layer 1 according to
the present embodiment.
FIG. 6 is a cross-sectional view illustrating a structure from the
buffer layer 6 to a tunnel barrier layer 3 on a semiconductor
substrate 5.
The buffer layer 6, the underlying layer 4, the storage layer 1,
and the tunnel barrier layer 3 are sequentially formed on a
semiconductor substrate 5.
The buffer layer 6 has a structure in which a first buffer layer 61
and a second buffer layer (Ti layer 64) are stacked in this order
on the semiconductor substrate 5. The first buffer layer 61 is
formed of W, Ta, Hf, or the like. The thickness of the first buffer
layer 61 is approximately 10-100 .ANG..
The Ti layer 64 should preferably be polycrystalline, and has a
thickness of approximately 10-20 .ANG..
The underlying layer 4 includes an MgO layer formed on the Ti layer
64 of the buffer layer 6. The MgO layer is formed of MgO in an
oxygen-deficient state, and contains oxygen in an amount less than
that of MgO with a stoichiometric composition. That is, an amount
of oxygen per unit volume contained in the MgO layer of the
underlying layer 4 is less than that of the MgO forming the tunnel
barrier layer 3.
The storage layer 1 has a structure in which a first storage layer
(CoFeB layer) 11, an insertion layer (W or Ta layer) 12, and a
second storage layer (CoFeB layer) 13 are stacked in this order on
the underlying layer 4, as in the first embodiment. The CoFeB
layers 11, 13 are formed of (CoFe.sub.50-100)B.sub.10-20. That is,
the CoFeB layer 11 has a composition in which B is contained at a
ratio of 10-20 at %, and a ratio of Co to Fe falls within the range
of 0.5:0.5 to 0:1. The thickness of the CoFeB layer 11 is
approximately 5-20 .ANG.. The thickness of the CoFeB layer 13 is
approximately 5-15 .ANG.. The thickness of the insertion layer 12
is approximately 1-5 .ANG.. In the above-described example, the
storage layer 1 has a stacked structure; however, the storage layer
1 may also be a single CoFeB layer.
A tunnel barrier layer 3 formed of an MgO layer is arranged on the
CoFeB layer 13 of the storage layer 1.
By using the structure shown in FIG. 6, perpendicular magnetic
anisotropy can be generated in the CoFeB layers 13, 11 in the
vicinity of an interface between the MgO layers 3, 4 provided on
both surface sides of the storage layer 1 and the storage layer 1.
Further, the resistance of the underlying layer (MgO layer) 4 can
be reduced to 0.5 .mu..OMEGA./cm.sup.2 or less.
In the present embodiment, an MgO layer in an oxygen-deficient
state is formed as an underlying layer 4 on a Ti layer 64 of a
buffer layer 6. Thereby, the series resistance of the underlying
layer (MgO layer) 4 arranged between the buffer layer 6 and the
storage layer 1 can be greatly reduced.
Effect
In a conventional MTJ having perpendicular magnetic anisotropy, the
thickness of a CoFeB layer must be 1 nm or less, in order to
generate perpendicular magnetic anisotropy in a storage layer. This
is because, since interface magnetic anisotropy is derived only
from an interface between an MgO tunnel barrier layer and a CoFeB
storage layer, perpendicular magnetic anisotropy energy of the
entire storage layer rapidly decreases as the film thickness of the
CoFeB increases.
A structure has been proposed in which an MgO layer is provided on
the opposite side of a tunnel barrier layer, a storage layer is
divided into two, and a non-magnetic layer of Ta, for example, is
interposed between CoFeB storage layers. When the MgO layer is used
as an underlying layer or a cap layer of the storage layer as in
this case, however, a high resistance of MgO, which functions as a
series parasitic resistance in an MTJ, can cause increase in
resistance and deterioration in magnetoresistance (MR) ratio.
To address this, the present embodiment uses W or Ti as a buffer
layer and forms MgO on the buffer layer, as described above.
Thereby, perpendicular magnetic anisotropy can be generated in the
storage layer to a sufficient degree, and the series resistance of
the underlying layer (MgO layer) can be greatly reduced. It has
been discovered that this is further more effective by making MgO
in an oxygen-deficient state.
FIG. 7 shows an example in which perpendicular magnetic anisotropy
energy is compared between the storage layer of the present
embodiment and a conventional storage layer. The horizontal axis
represents the thickness of the storage layer (CoFeB), and the
vertical axis represents the perpendicular magnetic anisotropy
energy of the storage layer.
In the conventional storage layer, perpendicular magnetic
anisotropy energy (Ku) rapidly decreases as the film thickness
increases. In the present embodiment, on the other hand, since
interface perpendicular magnetic anisotropy functions from both
surface sides of the storage layer, large perpendicular magnetic
anisotropy energy can be maintained even if the film thickness of
the storage layer is increased. Further, by increasing volume
components of the storage layer, retention properties can be
improved.
That is, magnetic anisotropy energy of the storage layer derived
from interface perpendicular anisotropy decreases as the film
thickness of the storage layer increases. Accordingly, the film
thickness of the storage layer cannot be increased. It is
advantageous for the retention properties of the storage layer, on
the other hand, to make the film thickness increased by volume
components and demagnetizing-field components. Accordingly, the
retention properties of the storage layer can be increased as the
film thickness dependence of Hk decreases in the term of Hk(t) in
the following formulas: Retention=Ku(t)*V(t)/kBT
Ku=Ms*t*Hk(t)/2-demagnetization term(t),
where K.sub.u denotes the perpendicular magnetic anisotropy, V
denotes the volume of a perpendicular magnetic material, kg denotes
the Boltzmann constant, T denotes the temperature of the
perpendicular magnetic material, M.sub.S denotes saturation
magnetization, t denotes the thickness of a storage layer, and Hk
denotes an anisotropy field.
Third Embodiment
In a third embodiment, a description will be given on a magnetic
random access memory (MRAM). The MRAM of the third embodiment has a
structure in which the magnetoresistive element according to the
first and second embodiments is used as a storage element.
FIG. 8 is a circuit diagram illustrating a configuration of the
MRAM according to the third embodiment.
The MRAM comprises a memory cell array 50 including a plurality of
memory cells MC arranged in a matrix pattern. In the memory cell
array 50, a plurality of pairs of bit lines BL, /BL are aligned so
as to extend in a column direction. Further, in the memory cell
array 50, a plurality of word lines WL are aligned so as to extend
in a row direction.
Each of a plurality of memory cells MC is arranged at an
intersection of the bit line BL and the word line WL. Each of the
memory cells MC includes a magnetoresistive element 100 and a
select transistor (such as n-channel MOS transistor) 51. One end of
the magnetoresistive element 100 is connected to the bit line BL.
The other end of the magnetoresistive element 100 is connected to
the drain of the select transistor 51. The source of the select
transistor 51 is connected to the bit line /BL. The gate of the
select transistor 51 is connected to the word line WL.
A row decoder 52 is connected to the word line WL. A write circuit
54 and a read circuit 55 are connected to the pair of bit lines BL,
/BL. A column decoder 53 is connected to the write circuit 54 and
the read circuit 55. Each of the memory cells MC is selected by the
row decoder 52 and the column decoder 53.
Writing of data to the memory cells MC is performed as will be
described below. In order to select a memory cell MC to which data
is written, a word line WL to be connected to the memory cell MC is
activated. Thereby, the select transistor 51 is turned on.
According to data to be written, a bi-directional write current Iw
is supplied to the magnetoresistive element 100. More specifically,
when a write current Iw is supplied to the magnetoresistive element
100 in a direction from left to right, a write circuit 54 applies a
positive voltage to the bit line BL, and a ground voltage to the
bit line /BL. When a write current Iw is supplied to the
magnetoresistive element 100 in a direction from right to left, the
write circuit 54 applies a positive voltage to the bit line /BL,
and a ground voltage to the bit line BL. Thereby, binary 0 or
binary 1 can be written to the memory cell MC.
Reading of data from the memory cell MC is performed as will be
described below. A select transistor 51 of a memory cell MC to be
selected is turned on. The read circuit 55 supplies a read current
Ir, which flows in a direction from right to left, for example, to
the magnetoresistive element 100, i.e., supplies a read current Ir
from the bit line /BL to the bit line BL. The read circuit 55
detects the resistance of the magnetoresistive element 100 on the
basis of the read current Ir. Further, the read circuit 55 reads
data stored in the magnetoresistive element 100 on the basis of the
detected resistance.
As described above, according to the present embodiment, by using
MgO as an underlying layer of a storage layer in an MTJ, interface
magnetic anisotropy can be generated from both of an MgO tunnel
barrier layer and an MgO underlying layer. Thereby, retention
properties can be greatly improved. Further, by making the MgO
underlying layer in an O-deficient state and forming the buffer
layer of W or Ti, the resistance of the MgO underlying layer can be
greatly reduced. Thereby, an MTJ having a high MR ratio can be
formed while using the MgO underlying layer.
Each of above-described MTJ structures can be introduced as MTJ
elements of memory cells. Memory cells, memory cell arrays and
memory devices is disclosed in U.S. patent application Ser. No.
13/420,106, Asao, the entire contents of which are incorporated by
reference herein.
While certain embodiments have been described, these embodiments
have been presented by way of example only, and are not intended to
limit the scope of the inventions. Indeed, the novel embodiments
described herein may be embodied in a variety of other forms;
furthermore, various omissions, substitutions and changes in the
form of the embodiments described herein may be made without
departing from the spirit of the inventions. The accompanying
claims and their equivalents are intended to cover such forms or
modifications as would fall within the scope and spirit of the
inventions.
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